A decrease in skeletal muscle blood flow is found in aging and is strongly correlated to an increase in efferent sympathetic nerve activity. Although both sexes experience these age-related changes, women experience a greater magnitude increase in sympathetic nerve activity and blood pressure compared to men. Previous work has focused on age-related changes in post-junctional sympathetic receptors, but there is intriguing new evidence that there are sex-related differences in pre-junctional sympathetic neurotransmission. This opens the possibility that aging of sympathetic neurotransmission may also be unique in females. Recent advances in biosensor technology allows simultaneous real-time recording of neurotransmitters and permits a detailed study of release, pre-junctional, and metabolic mechanisms with age and sex. The overall goal of this proposal is to identify important mechanistic changes in sympathetic neurotransmission in females through adulthood and into senescence. The underlying hypothesis in this proposal is that the age- related increase in vascular resistance in females is, in part, caused by an increase in biologically available sympathetic neurotransmitters. This hypothesis will be systematically analyzed under the following specific aims: 1. To determine whether there are age-related changes in neurotransmitter release and pre- junctional receptor modulation. 2. To determine whether there are age-related changes in the breakdown of neurotransmitter in the synapse. The purpose of this proposal is to study an under-investigated area of aging that may have an impact on blood pressure management in females. The experimental approach to be employed is a combination of video microscopy with biosensor technology and specific adrenergic and non-adrenergic agonists and antagonists for investigation of the vessels subserving the hindlimb. This will be the first time that a novel ATP biosensor technology will be applied to vascular tissue. These data are the beginning of the novel story concerning senescence, sex, sympathetic neurotransmission, blood flow, and blood pressure control. This information has potential to impact the management of age-related decline in blood flow in females and ultimately improve exercise tolerance and blood pressure management. PUBLIC HEALTH RELEVANCE: A decrease in skeletal muscle blood flow is found in aging and is strongly correlated to an increase in efferent sympathetic nerve activity. Although both sexes experience these age- related changes, women experience a greater magnitude increase in sympathetic nerve activity and blood pressure compared to men. The experiments in this proposal are the beginning of the novel story that has the potential to impact the management of age-related decline in blood flow in females and ultimately improve exercise tolerance and blood pressure management.